Boolean union keeps internal coplanar interface faces

[darwin]

Summary

When two solids touch on a coplanar face, boolean_union keeps the shared interface as two opposite-facing faces instead of removing it.

This creates an internal partition inside the union result:

• topology is non-manifold (junction edges have more than 2 incident faces)
• surface_area is overcounted
• volume can still appear correct

Minimal reproduction

[union [cube 10 10 25] [translate 0 0 25 [cube 10 10 35]]]

Two cuboids are stacked and share the entire plane z = 25.

Expected

Result should be identical to a single 10 x 10 x 60 cuboid:

• no internal faces
• manifold boundary
• surface_area = 2600

Actual

• result contains 12 faces
• two extra faces lie on the internal interface plane (z = 25), with opposite normals
• surface_area = 2800 (+200 = 2 x (10 x 10))

Why this is a bug

Keeping both sides of the mating coplanar interface means interior geometry is emitted as boundary geometry.

That is invalid for a closed solid boundary representation:

• internal partitions must not appear in the outer shell
• each boundary edge should be shared by exactly 2 boundary faces

Impact

• non-manifold output from a union operation
• incorrect surface_area
• can break downstream CAD/mesh workflows that require valid closed solids

Larger repro (partial overlap)

[union [cube 50 30 5] [translate 0 0 5 [cube 5 30 40]]]

Expected clean L-shape boundary.

Actual result includes two spurious internal faces at z = 5 in the column footprint (x = 0..5, y = 0..30), producing non-manifold junction edges.

Reference test

Related test on GitHub:

• test_union_coplanar_no_internal_faces

Inline test code (self-contained in this report):

#[test]
fn test_union_coplanar_no_internal_faces() {
    let bottom = make_cube(10.0, 10.0, 25.0);
    let mut top = make_cube(10.0, 10.0, 35.0);
    translate_brep(&mut top, 0.0, 0.0, 25.0);

    let result = boolean_op(&bottom, &top, BooleanOp::Union, 32);

    // Volume should be correct: 10 * 10 * 60 = 6000
    let mesh = result.to_mesh(32);
    let volume = compute_mesh_volume(&mesh);
    assert!(
        (volume - 6000.0).abs() < 10.0,
        "Union volume should be 6000, got {:.1}",
        volume
    );

    // The outer shell must have exactly 6 faces (no internal partition)
    let brep = result
        .as_brep()
        .expect("coplanar box union should produce BRep, not mesh fallback");
    let solid = &brep.topology.solids[brep.solid_id];
    let shell = &brep.topology.shells[solid.outer_shell];
    assert_eq!(
        shell.faces.len(),
        6,
        "Expected 6 outer faces for a 10x10x60 cuboid, got {} \
         (internal coplanar faces not removed)",
        shell.faces.len()
    );
}

Notes

• Coplanar side-face splitting by itself is cosmetic.
• The critical defect is retention of the internal coplanar interface faces.